A blocked polyisocyanate composition, as well as a melamine curing agent, has been widely used as a thermal-crosslinking-type curing agent in baking finish. In recent years, it has been pointed out that formalin is generated when a melamine curing agent is used. From the viewpoint of the global environment, safety, sanitation, and the like, a blocked polyisocyanate composition has attracted attention. Conventionally known blocking agents used for such a blocked polyisocyanate composition include oximes, phenols, alcohols, and lactams. However, since a blocked polyisocyanate composition formed using such a conventional blocking agent generally requires a high baking temperature of 140° C. or higher, energy costs become enormously high. In addition, there has been a limitation that such a blocked polyisocyanate composition that requires high-temperature baking cannot be used in the processing of plastic products having low heat resistance.
In order to overcome such a disadvantage, a pyrazole blocked polyisocyanate composition (Patent Literature 1) and an aliphatic secondary amine blocked polyisocyanate composition (Patent Literature 2) have been proposed as blocked polyisocyanate compositions that form a crosslinked coating film at a relatively low temperature. However, these blocked polyisocyanate compositions require a baking temperature of approximately 120° C., and thus, a further decrease in the baking temperature has been desired.
Blocked polyisocyanate compositions that have been proposed to enable such a further decrease in the baking temperature include: a coblocked polyisocyanate composition formed using (α) a diisopropylamine, (β) an active methylene compound, and (γ) an oxime as blocking agents (Patent Literature 3); a blocked polyisocyanate composition formed using a malonic diester as a blocking agent (Patent Literature 4); a blocked polyisocyanate composition formed using diethyl malonate and ethyl acetoacetate as blocking agents (Patent Literature 5); and a blocked polyisocyanate composition formed using isobutanoyl acetate as a blocking agent (Patent Literature 6).
Moreover, Patent Literature 7 describes a CH-active alkyl ester or an addition product formed by adding such a CH-active alkyl ester to isocyanate as a raw material for a synthetic intermediate of an amino group-containing curing component.